This invention relates to the field of multi-layer printed circuit board preparation. More particularly, this invention relates to processing of a multi-layer circuit board after lamination and prior to drilling.
Multi-layer printed circuit boards have found increasing use in the manufacture of electronic products. A typical multi-layer circuit board comprises a plurality of individual circuit boards laminated together. Each printed circuit board comprises an electrically non-conductive substrate material having conductive patterns formed on one or both sides thereof. The conductive patterns are used to provide interconnect paths among the active and passive electronic components eventually mounted on the multi-layer printed circuit board. In order to afford proper connections, predetermined points on the conductive paths on different individual boards must be interconnected, and these interconnections are typically made by drilling holes through the board at precise locations, followed by plating the through-holes with an electrically conductive material such as solder. Since there are literally hundreds of such holes to be drilled in a typical multi-layer board, the drilling is typically done in an automatic drilling apparatus controlled by a computer program. A typical drilling apparatus used for this purpose has multiple drill spindles which are independently activated by the master program in order to reduce the total drilling time required to form the multiple holes on a single multi-layer board. In order to properly position the multi-layer board initially in the drilling apparatus, some fixed referencing arrangement is usually employed, such as registration holes formed in edge portions of the board which mate with pins carried by the drilling apparatus table.
A severe problem encountered in the multi-layer board fabrication art is that of misregistration among the individual boards comprising the assembly. While the individual board patterns can be formed very precisely using conventional photolithography, exact registration among the multiple patterns on the several boards is impossible to achieve due to distortions introduced during the lamination processing. These distortions typically lead to maximum misregistration at the outer edges of the panels. The principal criterion for a useful board can be simply stated: each hole drilled through the multi-layer board must be surrounded by a conductive material at each layer in order to form a useful hole. Due, however, to the misregistration introduced during the lamination process, this criterion cannot be met by all multi-layer boards. In fact, the rejection rate for multi-layer boards has a present practical range of from 5% to 20%, depending upon the minimum pattern line width, maximum acceptable hole diameter, and pattern complexity.
In the past, attempts at quality control for multi-layer circuit boards have centered about an inspection process wherein the developing multi-layer board is photographed at preselected stages of the fabrication process using an x-radiation source and x-radiation sensitive film. After the films are developed, the successive photographs are compared to discern the degree of misregistration or distortion introduced during the intervening steps between the preselected stages. Once the final multi-layer assembly is completed, and before the board is subjected to programmed drilling, a final comparison is made and the board is either accepted or rejected for drilling based upon this final comparison. Although useful, this process is slow and cumbersome and can only be effectively employed to sample representative multi-layer boards with theoretically identical patterns, which are undergoing multi-layer lamination. Since this technique is only amenable to spot sampling in a production environment, many multi-layer boards which should be rejected for misregistration or deformation are passed on to the automatic drilling station, where they are uselessly drilled and ultimately scrapped. The automated drilling of a board which does not meet the minimum registration requirements is extremely wasteful, since it results in a product which cannot be used. With relatively dense boards, thousands of holes are actually drilled, which consumes relatively large periods of the drilling machine time. For example, in an 18 inch .times.24 inch multi-layer board, the number of holes typically ranges from 12,000 to 14,000, and the complete drilling of such a board can take as long as 90 minutes. As a result, the x-ray film inspection process has not been found to be a satisfactory solution to the problem of effective quality control for multi-layer boards prior to drilling.
In an effort to avoid the disadvantages with the x-radiation photographic process, a system has been developed to permit on-line inspection of multi-layer boards using an x-ray imaging system which examines test holes formed near the corners of the individual board layers and displays the percent of registration among all corresponding holes in a given corner region. While useful, this system is very large and expensive, and merely provides a percent registration figure for each set of test holes on a sequential basis. If a given multi-layer board falls within the permitted percentage of misregistration, it will be passed on for drilling. For those accepted boards which are close to the maximum permitted percentage misregistration, the accumulation of tolerance errors inherent in the drilling machine can result in a multi-layer board with unacceptable through holes.